In electronic devices such as DRAM (Dynamic Random Access Memory), the structures of wires and electrodes are becoming finer and more complex to achieve higher performance, and the improvement of accuracy is now desired for the shapes of these devices.
To form electrodes and wires in electronic devices, in general, trenches are formed at positions where wires or electrodes are to be formed on a substrate, a metal material which will become wires or electrodes is buried in the trenches, and excess is removed by chemical mechanical polishing or the like.
Copper which has high conductivity has been widely used as an electrode material or a wire material which is buried in the trenches. To bury copper in the trenches, a plating method is advantageous because it has an advantage that copper can be buried even in trenches having a high aspect ratio at a high filling rate (refer to JP-A 2000-80494 and JP-A 2003-318258).
When a substrate having the trenches is an insulator having no conductivity (such as a substrate made of silicon oxide), a conductive layer (seed layer) which is a primary coat for plating must be formed on the surface of the substrate prior to plating. It is known that when copper comes in contact with an insulator typified by silicon oxide, a phenomenon (generally called “migration”) that a copper atom moves to the insulator from the copper layer occurs. When the migration of the copper atom occurs at the interface between copper and the insulator in an electronic device, the electric properties of the device are impaired. Therefore, a barrier layer must be formed between the insulator and copper of the electronic device.
In recent years, there has been proposed a technology for attaining the above object by using cobalt as a material which serves as a seed layer for burying copper into trenches by plating and also as a barrier layer at the interface between the insulator and copper and by using a special chemical vapor deposition method (refer to JP-A 2006-328526). In the technology disclosed by the above publication, a first substrate on which a cobalt film is to be formed and a second substrate having a cobalt precursor thereon are arranged close to each other and opposed to each other, and a cobalt precursor sublimating from the second substrate is supplied onto the first substrate and converted into cobalt on the first substrate so as to form a cobalt film. The above publication discloses octacarbonyl dicobalt as the cobalt precursor. With this technology, a cobalt film which is uniform in thickness even in the inside of each trench and has high adhesion can be easily formed even when a substrate having trenches with a high aspect ratio is used. Therefore, the above object is attained.
However, it is known that an ordinary cobalt precursor such as octacarbonyl dicobalt has a storage stability problem that it is gradually converted into a stable complex having low sublimation properties during storage. When this cobalt complex including a stable complex is used as the precursor in the chemical vapor deposition method, the stable complex does not sublimate and remains. Therefore, it is necessary to prepare the precursor used for the chemical vapor deposition method in an amount which is much larger than its theoretical value calculated from the weight of the cobalt film to be formed. Since the above residue cannot be reused as the cobalt precursor in the chemical vapor deposition method, the formation of the cobalt film by the chemical vapor deposition method becomes excessively costly.